Formation and lithium doping of graphene on the surface of cobalt silicide

The intercalation of silicon under graphene on the Co(0001) surface, which is accompanied by the formation of a silicon solid solution in cobalt and by the formation of a surface crystalline Co₂Si phase, has been investigated using photoelectron spectroscopy. It has been shown that the formation of cobalt silicide leads to a substantial weakening of the hybridization of electronic states of graphene and cobalt and to the recovery of the Dirac spectrum of electronic states of graphene near the Fermi level. This has made it possible to investigate the electron doping of graphene on the cobalt silicide substrate upon deposition of lithium on its surface. It has been found that doping with lithium leads to a significant charge transfer onto graphene, and the electron concentration reaches 3.1 × 10¹⁴ cm^?2. Moreover, the specific form of the Fermi surface creates favorable conditions for the enhancement of the electron-phonon coupling. As a result, the formed system can be considered as a candidate for the creation of superconductivity in single-layer graphene.     

Modification of the electronic structure of graphene by intercalation of iron and silicon atoms

The ab initio calculations of the electronic structure of low-dimensional graphene–iron–nickel and graphene–silicon–iron systems were carried out using the density functional theory. For the graphene–Fe–Ni(111) system, band structures for different spin projections and total densities of valence electrons are determined. The energy position of the Dirac cone caused by the p _z states of graphene depends weakly on the number of iron layers intercalated into the interlayer gap between nickel and graphene. For the graphene–Si–Fe(111) system, the most advantageous positions of silicon atoms on iron are determined. The intercalation of silicon under graphene leads to a sharp decrease in the interaction of carbon atoms with the substrate and largely restores the electronic properties of free graphene.

     

Sodium Intercalation of Graphene Films on Re( $$10\bar 10$$ 10 1 ¯ 0 )

It is shown that during low-temperature (300–500 K) intercalation of sodium atoms into thin multilayer graphene and graphite films on rhenium the first graphene layer plays the role of a trap to which atoms coming on the surface diffuse through a graphite film. The intercalation phase of the interlayer space in the graphite bulk is actively filled at a sodium atoms concentration under the first graphene layer close to the maximum possible (2 ± 0.5) × 10¹⁴ cm^–2. This phase capacity is proportional to the graphite film thickness that can be varied in this work from one graphene layer to ~50 atomic layers. The diffusion energy E _d of Na atoms through the graphite film was estimated to be E _d ≈ 1.4 eV.

     

Magnetoelectric oxide films for spin manipulation in graphene

The challenge of creating a graphene spin field effect transistor (spin‐FET) demands a magnetic gate dielectric material whose magnetization can be switched electrically. We have grown films of Cr₂O₃ on top of graphite and graphene by pulsed laser deposition that shows this crucial functionality. We demonstrate that the Cr₂O₃ films are magnetoelectric by poling them in combined electric and magnetic fields and then using magnetic force microscopy to observe spontaneous surface domain structure as a function of poling field. In addition, we show that the electric field created by a conducting AFM tip can be used to write magnetic patterns in the film that demonstrate the kind of continuous magnetoelectric control needed for a prototype spin‐FET. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Formation and magnetic properties of the silicon-cobalt interface

Formation of the Si/Co interface and its magnetic properties have been studied by high-resolution photoelectron spectroscopy with synchrotron radiation. The experiments have been performed in situ in superhigh vacuum (5 × 10^?10 Torr) with coating thicknesses up to 2 nm. It has been found that, in the initial stage of silicon deposition on the surface of polycrystalline cobalt maintained at room temperature, ultrathin layers of the Co₃Si, Co₂Si, CoSi, and CoSi₂ silicides are formed. The three last phases are nonmagnetic, and their formation gives rise to fast decay of magnetic linear dichroism in photoemission of Co 3p electrons. At deposition doses in excess of ～0.4 nm Si, a film of amorphous silicon grows on the sample surface. It has been established that the Si/Co interphase boundary is stable at temperatures up to ～250°C and that further heating of the sample brings about escape of amorphous silicon from the sample surface and initiates processes involving silicide formation.     

Growth of magnetic materials and structures on Si(0 0 1) substrates using Co2Si as a template layer

We report in this paper the use of Co₂Si silicide as a template layer for the integration of magnetic materials and structures on silicon substrate. By undertaking Co deposition on silicon at a temperature of about 300 °C, we show that it is possible to obtain a smooth and epitaxial Co₂Si layer, which can act as a template layer preventing the reaction between Co and other transition metals with silicon. Two examples of over-growth of magnetic materials and structures on this template layer will be presented: growth of ferromagnetic Co layers and of magnetic tunnel junctions (Co(Fe)/AlO_x/NiFe).     

Significant positive magnetoresistance of graphene/carbon composite films prepared by electrospraying and subsequent heat treatment

Graphene/carbon composite films were prepared by electrospraying a graphene/polyacrylonitrile composite solution on SiO₂-coated silicon substrates and subsequent heat treatment. The as-produced graphene/carbon composite films had a porous structure comprising graphene layers. With a magnetic field applied perpendicularly to the sample, an unexpectedly significant positive magnetoresistance attributed to e–e interaction and weak localization has been observed, which constantly increases with the magnetic field in the temperature range of 300–50 K from 0 to 80 kOe.     

Intercalation of Iron Atoms under Graphene Formed on Silicon Carbide

The intercalation of iron under a graphene monolayer grown on 4H-SiC(0001) is studied. The experiments have been carried out in situ under conditions of ultrahigh vacuum by low-energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and near carbon K-edge X-ray absorption spectroscopy. The deposited iron film thicknesses have been varied within 0.1–2 nm and the sample temperatures from room temperature to 700°C. It is shown that the intercalation process begins at temperatures higher than ~350°C. In this case, it is found that intercalated iron atoms are localized not only between graphene and a buffer layer coating SiC, but also under the buffer layer itself. The optimal conditions of the intercalation are realized in the range 400–500°C, because, at higher temperatures, the system becomes unstable due to the chemical interaction of the intercalated iron with silicon carbide. The inertness of the intercalated films to action of oxygen is demonstrated.

     

Direct growth of graphene film on piezoelectric La3Ga5.5Ta0.5O14 crystal

For the first time, a few layer graphene was grown on the surface of the polar X ‐cut (110) of a piezoelectric La₃Ga_5.5Ta_0.5O₁₄ crystal by the CVD method. This polar X ‐cut is characterized by a good matching of the crystal lattice parameters of La₃Ga_5.5Ta_0.5O₁₄ and two‐dimensional graphene crystal, as well as the presence of piezoelectric fields on the surface of the substrate, which could affect the graphene growth process. Raman spectroscopy investigation has shown the ability for direct growth of graphene on the piezoelectric crystal. The NEXAFS spectroscopy studies of the film grown on the surface of the X ‐cut of an La₃Ga5_.5Ta_0.5O₁₄ crystal also confirmed that the grown film is graphene. Moreover, the NEXAFS spectra enable the conclusion that additional electron states are formed as a result of chemical bonding between the atoms of graphene and the substrate which proceeds through hybridization of the valence electron states of the substrate and graphene atoms. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

同步辐射光激励的二氧化硅薄膜刻蚀研究

利用热氧化法在硅晶片上生长SiO₂薄膜，结合光刻和磁控溅射技术在SiO₂薄膜表面制备接触型钴掩模，通过掩模方法在硅表面开展了同步辐射光激励的表面刻蚀研究，在室温下制备了SiO₂薄膜的刻蚀图样.实验结果表明：在同步辐射光照射下，通入SF₆气体可以有效地对SiO₂薄膜进行各向异性刻蚀，并在一定的气压范围内，刻蚀率随SF₆气体浓度的增加而增加，随样品温度的下降而升高；如果在同步辐射光照射下，用SF₆和O₂的混合气体作为反应气体，刻蚀过程将停止在SiO₂/Si界面，即不对硅刻蚀，实现了同步辐射对硅和二氧化硅两种材料的选择性刻蚀；另外，钴表现出强的抗刻蚀能力，是一种理想的同步辐射光掩模材料. 关键词： 同步辐射刻蚀 接触型钴掩模 二氧化硅薄膜     

Synthesis and investigation of titanium diselenide intercalated with ferrocene and cobaltocene

Single crystals and polycrystals of titanium diselenide TiSe₂ intercalated with ferrocene Fe(η⁵-C₅H₅)₂ and cobaltocene Co(η⁵-C₅H₅)₂ are synthesized. The magnetic susceptibility and electrical resistivity of the intercalation compounds are measured. The results obtained demonstrate that the intercalation brings about the formation of an impurity band with a temperature-dependent width.     

Probing disorder and charged impurities in graphene by Raman spectroscopy

Disorder and doping can strongly affect the properties of graphene. Here we analyze these effects on several samples by Raman spectroscopy. In particular, we show that pristine and unprocessed graphene samples deposited on silicon, covered with a thin silicon oxide layer, show strong variations in their Raman spectra, even in absence of disorder. The variation in the Raman parameters is assigned to charged impurities. This shows that as‐deposited graphene is unintentionally doped, reaching charge concentrations up to 10¹³ cm^–2 under ambient conditions. The doping varies from sample to sample and the charges are inhomogeneously distributed on a submicron scale. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ordered magnetic multilayer nanobowl array by nanosphere template method

Ordered magnetic multilayer [Co/Pt]_n nanobowls have been fabricated over a silicon substrate based on a polystyrene (PS) monolayer film. The ordered PS monolayer was first prepared by the self-assembly technique, which was used as the template for the multilayer film [Co/Pt]_n deposition. The ordered magnetic multilayer [Co/Pt]_n nanobowl array was obtained after the transferring and the selective etching process. The nanobowls show a uniform size and smooth surfaces. The nanobowls stuck to the neighbors and notches were observed in the bowl brims because of the contact points between the closed-packed PS beads. The nanobowls could be separated from their neighbors by thinning the PS beads before the film deposition and no notches were observed anymore. Compared to the chemical method, this method showed more flexible choices of the material to fabricate the nanobowls, which extended the application scope of the nanobowls greatly.     

Electrochemical formation of intermediate layer for Co/Pd multilayered media

Pd nanocluster seeds were formed on a soft magnetic underlayer (SUL) using an electrochemical substitution reaction, and were utilized as an intermediate layer for a Co/Pd multilayered ([Co/Pd]_n) perpendicular magnetic recording medium. A CoNiFeB film prepared with electroless deposition was used as SUL, which was immersed into a PdCl₂ solution for the formation of Pd seeds. The Pd seeds were found to effectively reduce the size of magnetic domains in the [Co/Pd]_n film deposited on them. The optimization of the concentration of the PdCl₂ solution and the use of the pretreatment process with a SnCl₂ solution were effective to obtain the smooth SUL surface with fine Pd seeds as small as 5 nm. The 20 nm-thick [Co/Pd]_n film deposited on the optimized Pd seeds/CoNiFeB SUL exhibited a high coercivity of 7.8 kOe and a small magnetic domain size of 69 nm. These results indicated that the combination of the Pd seeds and the electroless-deposited SUL was desirable in terms of the improvement not only in the magnetic properties of [Co/Pd]_n media but also in the mass productivity of the underlayer.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

Surface modification of exchange-coupled Co/NiOx magnetic bilayer by bias sputtering

We have investigated the effect of bias voltage on sheet resistance, surface roughness and surface coverage of Co/NiO_x magnetic bilayer. In addition, interface topography and corrosion resistance of the Ta/Co/Cu/Co/NiO_x/Si(1 0 0) system have been studied for Co layers deposited at an optimum bias voltage. Atomic force microscopy (AFM) and four point probe sheet resistance (R_s) measurement have been used to determine surface and electrical properties of the sputtered Co layer at different bias voltages ranging from 0 to −80 V. The Co/NiO_x bilayer exhibits a minimum surface roughness and low sheet resistance value with a maximum surface coverage at V_b=−60 V resulted in a slight increase of magnetic resistance and its sensitivity for the Co/Cu/Co/NiO_x/Si(1 0 0) magnetic multilayers, as compared with the same magnetic multilayers containing unbiased Co layers. The presence of Ta protection layer improves the corrosion resistance of the multilayers by three orders of magnitude in a humid environment.     

Intercalation of metals and silicon at the interface of epitaxial graphene and its substrates

Intercalations of metals and silicon between epitaxial graphene and its substrates are reviewed. For metal intercala- tion, seven different metals have been successfully intercalated at the interface of graphene/Ru（O001） and form different intercalated structures. Meanwhile, graphene maintains its original high quality after the intercalation and shows features of weakened interaction with the substrate. For silicon intercalation, two systems, graphene on Ru（O001） and on Ir（l I 1）, have been investigated. In both cases, graphene preserves its high quality and regains its original superlative properties after the silicon intercalation. More importantly, we demonstrate that thicker silicon layers can be intercalated at the interface, which allows the atomic control of the distance between graphene and the metal substrates. These results show the great potential of the intercalation method as a non-damaging approach to decouple epitaxial graphene from its substrates and even form a dielectric layer for future electronic applications.     

Stress relaxation of GaN microstructures on a graphene‐buffered Al2O3 substrate

GaN microstructures were grown on c‐Al₂O₃ with a multi‐stacked graphene buffered layer using metal metal‐organic chemical‐vapor deposition. Under the same growth conditions, the nucleation of GaN was suppressed by the low surface energy of graphene, resulting in a much lower density of microstructures relative to those grown on c‐Al₂O₃. Residual stress in the GaN microstructures was estimated from the peak shift of the E₂ phonon using micro‐Raman spectroscopy. The results showed that the compressive stress of approximately 0.36 GPa in GaN on c‐Al₂O₃ caused by lattice mismatch and the difference in the thermal expansion coefficient was relaxed by introducing the graphene layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quasi‐Freestanding Graphene on SiC(0001) by Ar‐Mediated Intercalation of Antimony: A Route Toward Intercalation of High‐Vapor‐Pressure Elements

A novel strategy for the intercalation of antimony (Sb) under the $\text{(6}\sqrt{\mathrm{3}}\times \mathrm{6}\sqrt{\mathrm{3}})R\text{30}$° reconstruction, also known as buffer layer, on SiC(0001) is reported. Using X‐ray photoelectron spectroscopy, low‐energy electron diffraction, and angle‐resolved photoelectron spectroscopy, it is demonstrated that, while the intercalation of the volatile Sb is not possible by annealing the Sb‐coated buffer layer in ultrahigh vacuum, it can be achieved by annealing the sample in an atmosphere of Ar, which suppresses Sb desorption. The intercalation leads to a decoupling of the buffer layer from the SiC(0001) surface and the formation of quasi‐freestanding graphene. The intercalation process paves the way for future studies of the formation of quasi‐freestanding graphene by intercalation of high‐vapor‐pressure elements, which are not accessible by previously known intercalation techniques, and thus provides new avenues for the manipulation of epitaxial graphene on SiC.     

Magnetic behavior of nanocrystalline CoFe2O4

Magnetic nanoparticles of CoFe₂O₄ have been synthesized under an applied magnetic field through a co-precipitation method followed by thermal treatments at different temperatures, producing nanoparticles of varying size. The magnetic behavior of these nanoparticles was investigated. As-grown nanoparticles demonstrate superparamagnetism above the blocking temperature, which is dependent on the particle size. One of the nanoparticles demonstrated a constricted magnetic hysteresis loop with no or small coercivity and remanence at low magnetic field. However, the loop opens up at high magnetic field. This magnetic behavior is attributed to the preferred Co ions and vacancies arrangements when the CoFe₂O₄ nanoparticles were synthesized under an applied magnetic field. Furthermore, this magnetic property is strongly dependent on the high temperature heat treatments that produce Co ions and vacancies disorder.